1. Field of the Invention
The present invention relates to methods and systems for lithographic processing of a device, e.g. in electronic device processing especially semiconductor processing. More particularly, the present invention relates to lithographic attenuated phase shift masks, methods of making them and methods of using lithographic attenuated phase shift masks.
2. Description of the Related Technology
Lithographic processing is one of the key features in the manufacturing of today's circuits and integrated components. The quality of the lithographic processing is strongly dependent on the lithographic system and the mask used. Different types of lithographic masks are known such as binary masks, like chrome on glass masks and phase shift masks, such as e.g. alternating aperture phase shift masks (AAPSM) and attenuated phase shift masks (att. PSM). As is the case for binary masks (BIM), alternating phase shift masks consist of a substrate which is transparent for the wavelength of the light used during lithographic processing. On a surface of this transparent substrate, opaque areas are formed which block the propagating of light. However, contrary to binary masks where all light transmitting areas impact the light propagation in a similar intensity and phase, transmitting areas on an alternating phase shift mask are designed to provide an approximately 180° phase difference between light propagating through these transmitting areas and through other transmitting areas on the mask while preserving the intensity of the propagated light. A feature on mask, i.e. on opaque area, is always surrounded by transmitting areas of different phase. The occurrence of the phase difference leads to destructive interference, resulting in a sharp dark image. As is the case for alternating phase shift masks, also for attenuated phase shift masks a 180° phase difference is created between light propagating through different areas on the mask. However in case of an attenuated phase shift, this phase difference is created between a transmitting area and the opaque area. To this extent the material of the opaque area of the binary and alternating phase shift masks is replaced by halftone materials, i.e. a material or stack of materials that is partially transmissive for the wavelength of the light used during lithographic processing. A part of the incident light will be absorbed by the halftone material and a part of the light will be transmitted through the halftone material. FIG. 1 illustrates the orthogonal exposure of a device (200) comprising a substrate (220) covered with a photosensitive layer (210) through an attenuated phase shift mask (100) comprising a transparent substrate (110). On a surface of this transparent substrate (110) transmissive areas (130) and absorbing areas (120) are present. The intensity and phase of the light beams (320) in the absorbing areas (120) is changed with respect to the intensity and phase of the light beams (310) in the transmissive areas (130) as indicated by the respective wave curves showing amplitude (A) as function of time (t). The amount of light propagating through these halftone areas (120) is insufficient to make the photosensitive resist in corresponding areas on the wafer developable. The occurrence of the phase difference between the light with full intensity (330), i.e. propagating through the transmissive areas, and light with reduced intensity and phase shift (340), i.e. propagating through the halftone areas, will lead to destructive interference, resulting in an improved resolution. In FIG. 1 this is illustrated by the reduced and negative amplitude of the light (340) propagating through the halftone film compared with the substantially unaffected positive amplitude of the light (330) propagating through the transmissive areas (130). Such a halftone thin film can be made from bilayer films such as chromium (Cr) and silicon-oxynitride (SiON), tantalum (Ta) and silicon dioxide (SiO2), or of a single layer of molybdenum silicon (MoSi) or molybdenum silicide (MoSi2). These halftone layers should be as thin as possible to reduce manufacturing complexity and formation of slanted profiles of the halftone features. Currently molybdenum silicon is the preferred halftone material as it only requires processing of a single layer of material to form the absorbing areas (120) providing the required attenuating and phase shift. U.S. Pat. No. 5,869,212 discloses a method for manufacturing attenuated phase shift mask.
Due to down-scaling of technology, the features on mask will have smaller dimensions. Light propagating through the mask will become more diffracted such that less light is collected by the lens system of a lithographic tool and the obtainable resolution is reduced. In order to improve the resolution of optical lithography, lithographic systems with high numerical aperture (NA) are under development. While commercially available advanced dry lithograph systems can have a NA of about 0.9 but at the expense of an exceptionally high quality lens material, wet lithographic systems are under development offering a NA of 1 or higher. Such lithographic systems are often labelled hyper NA systems. In wet lithography, better known as immersion lithography, a liquid having a refractive index greater than 1 is present between the lens system and the substrate which is to be exposed. The presence of this immersion liquid will further improve the resolution of the system as it allows light to impinge on the exposed substrate at a larger angle.
A disadvantage of using state-of-the-art attenuated phase shift mask in hyper NA lithography is that these masks do not provide printed images with the required quality for advanced technologies. With increasing numerical aperture the image contrast image in the resist reduces, as illustrated by table 1 below for state-of-the-art attenuated halftone materials on a quartz substrate. The parameter image contrast is an image metric which is defined as the ratio of the difference between the maximum and minimum intensities to the sum of the maximum and minimum intensities in an image as obtained by projection of a mask pattern on photoresist coated substrate by a lithographic system.
TABLE 1Image contrast24.5 nm Cr/21 nm Ta/in resist68 nm MoSi99.2 nm SiON144 nm SiO2NA 0.850.620.600.64NA 1.200.540.460.58
Hence there is a need to have useful att. PSM, e.g. in view of the evolution towards high numerical aperture or hyper numerical aperture lithography systems and the associated shrinkage of features sizes and pitches, which puts stronger demands on the process window of associated lithographic processes.